1. To be glad to see someone leave (usually because the speaker dislikes the person). Ed has been driving me nuts with requests, so I'll be glad to see the back of him when the construction is done on his office.

2. To be glad to be rid of or finished with something, especially an unwanted or troublesome thing. Yeah, I'll be glad to see the back of this diet. Thankfully it's just another three weeks until the wedding!